There have been known display systems that form a composite three-dimensional image by sequentially emitting from an image source a series of two-dimensional images representing different depth planes of a three-dimensional subject. Each depth plane image is typically reflected by a mirror that is positioned to provide an optical path of predetermined length between the image source and an observer. Each depth plane image travels a different optical path length to create for the observer a composite light image having the appearance of depth. The resulting three-dimensional image has, therefore, full vertical, horizontal, and head-motion parallax. Display systems of this type are useful for examining three-dimensional images that consist of a series of visual planar data, such as, for example, ultrasound scans of tissue, wiring diagrams for multilayer printed circuit boards, air traffic control scans, and architectural plans.
Display systems that form the three-dimensional image by using mechanical means to change the position of the reflecting surface of a mirror to reflect depth plane images at different optical path locations are described in the article "Terminal puts three-dimensional graphics on solid ground,38 Electronics, 150-155 (July 28, 1981), by Stover. One system of this type employs a flat-plate mirror that repetitively moves back and forth along a straight line path. The mirror crosses plural locations along the path, each location corresponding to a different depth plane of a three-dimensional subject. Depth plane images emitted from an image surface are reflected from each location to an observer's eyes to produce the composite three-dimensional image. These display systems have proved to be mechanically impractical because of the precision required to match the instantaneous position of the mirror to the appearance of the corresponding depth plane image on the image surface.
A second system substitutes a varifocal mirror for the movable mirror described above. A varifocal mirror is one whose focal length changes with mechanical vibration. The vibration causes a continuous change in the shape (i.e., convex, flat, and concave) of the reflecting surface of the mirror. Changing the shape of the mirror creates the impression that images reflected by it originate from different distances from the observer. The mirror is vibrated in response to a signal that is synchronized with the appearance of depth plane images emitted from a cathode ray tube.
The varifocal mirror system suffers from a number of inherent drawbacks. First, the mirror continuously changes its shape and, as a consequence, eventually wears out. Second, the mechanical vibration causes a sinusoidal change in the focal length of the mirror. The varifocal mirror loses, therefore, the constancy of magnification obtainable with a flat-surface mirror. As a consequence, the images reflected from the mirror are deformed, thereby requiring compensation in the form of opposite deformation by pre-distortion of the images emitted from the cathode ray tube. Third, the vibrating mirror produces an unacceptable acoustic rumble because it behaves much like an audio speaker. The acoustic rumble can be prevented by maintaining the fundamental frequency of the mirror below 30 Hz, but this low frequency of reflection can cause flicker in the three-dimensional image. Fourth, the varifocal mirror system is not suitable as a color display. This is so because the persistence of phosphor emissions from a shadow mask cathode ray tube would cause a smearing of successive depth plane color images as the mirror continuously changes its focal length. To develop color images in a varifocal mirror system, one must terminate the phosphor emissions while the mirror changes its focal length to that corresponding to the next depth plane. Full color phosphors with the required persistence characteristics are not currently available.
A display system that relies on nonmovable flat-plate mirrors to form a three-dimensional light image is described in U.S. Pat. No. 4,190,856 to Ricks. The display system of Ricks employs an assembly of beam splitters or semitransparent mirrors in association with plural cathode ray tubes to form the three-dimensional image. The display system also includes at least one positive lens for repositioning the image toward an observer. Each cathode ray tube emits an image corresponding to a different depth plane. The images propagate concurrently through the semitransparent mirrors and combine on a common optical path to form a composite image. Since the images emitted from each cathode ray tube travel along different optical path lengths, the composite image appears to have depth.
This system suffers from the disadvantages of requiring plural cathode ray tubes and extensive electrical drive circuitry, thereby making a display system capable of developing numerous depth plane images quite large and expensive. In addition, an increase in the number of depth planes increases the optical path length required to develop a three-dimensional image. Increasing the optical path length limits the angle from which the image is viewable.
Another three-dimensional display system that uses plural cathode ray tubes and multidirectional beam splitters is described in the article "Multilayered 3-D display by multidirectional beam splitter," Applied Optics, Vol. 21, No. 20, 3659-3663 (Oct. 15, 1982), by Tamura and Tanaka. This structure also suffers from the disadvantage of a long optical path length and image degradation resulting from the thicknesses of the beam splitters.
A three-dimensional display system comprising an adapter that fits over the image face or screen of a single cathode ray tube is described in the article "Multilayer 3-D display adapter," Applied Optics, Vol. 17, No. 23, 3695-3696 (Dec. 1, 1978), by Tamura and Tanaka. The adapter employs semitransparent and fully-reflecting mirrors to produce optical paths of different lengths that develop the different image depth planes. In this display system, the optical path lengths are so large that the three-dimensional effect is diminished. In addition, since the adapter effectively divides the single-image face into several subfaces, each depth plane image, as well as the image window, is small.
A display system that uses a twisted nematic liquid crystal cell in association with a cholesteric liquid crystal layer to develop a two-dimensional image in a predetermined color is described in the article "Twisted nematic display with cholesteric reflector," J. Phys. D: Appl. Phys., Vol. 8, 1441-48 (1975), by Scheffer The single-color display receives linear polarized light rays of many wavelengths whose direction of polarization is selectively rotated by 0.degree. or 90.degree. by the twisted nematic cell. A quarter-wave plate positioned between the twisted nematic cell and the cholesteric layer receives the linearly polarized light rays passing through the twisted nematic cell and converts them to left- or right-circularly polarized light rays, depending upon the polarization direction of the incident light rays.
Within its reflection band of wavelengths, the cholesteric layer reflects circularly polarized light rays of the rotational sense of its helical twist and transmits circularly polarized light rays of the opposite rotational sense. Outside its reflection band, the cholesteric layer transmits light rays of all polarization states.
Scheffer describes the operation of a left-hand twist cholesteric layer as follows. Whenever the twisted nematic cell rotates the polarization direction of the incident light rays by 90.degree., left-circularly polarized light rays are incident on the cholesteric layer which reflects the light rays in an iridescent color that characterizes the layer. The single-color light rays reflect back through the system to be viewed by an observer. Whenever the twisted nematic cell rotates the polarization direction by 0.degree., right-circularly polarized light rays pass through the cholesteric layer and strike an absorbing material, which provides a preferred background for the colored image previously reflected.
The Scheffer article describes only a technique for producing a single-color two-dimensional image with the use of a twisted nematic cell and a cholesteric layer. The display system of Scheffer has not heretofore been suggested or adapted for use in either a monochrome or full color three-dimensional display system.